The invention is in the field of fiber optic technology, and particularly concerns the structure, operation, and manufacture off reflective notch couplers in an optical fiber. More specifically, the invention relates to the structure, operation, and manufacture of a fiber optic position sensor incorporating an optical fiber with a series of optical delay elements among which are interspersed a series of reflective optical couplers to form a time division multiplexing telemetry device.
Use of fiber optic technology to obtain an optical position signal from an optically-encoded surface is benefitted by an optical sensor structure which returns the optical sensor signal in the form of a multiplexed signal. Whether such a signal is time-multiplexed or positioned-multiplexed depends upon the structure of the fiber optic sensor. In this regard, it is asserted that optical position information is generated by means of the interaction of an optically-encoded surface which is illuminated in such a manner as to provide a set of digital optical signals indicative of the position of the surface. Such coding may be, for example, in the form of a pattern of reflective and non-reflective areas forming a Gray scale code. The surface is illuminated and a plurality of optical signals are developed from this illumination, the signals being either "on" or "off" according to whether the Grey scale bit is active or inactive. The plurality of optical signals making up the Gray scale code can be conducted in parallel through an equal number of optical fiber paths, each dedicated to transmission of one bit position of the code from the coded surface to a sensor processor. This arrangement is essentially "position-multiplexing" in which each code bit is represented by optical energy present or absent in a corresponding optical fiber.
Alternatively, the bits may be introduced into a single fiber in a time sequence corresponding with the magnitude sequence of the bits. Relatedly, this time-multiplexing of code bits requires a means for coupling the bits in a time sequence into the single fiber channel which conducts the bits from the coded surface to the sensor processor.
It will be evident to those skilled in the art that time-multiplexing of a set of digital signals into a single fiber could be provided by a single coupling mechanism which is mechanically scanned in a predetermined fashion across the optical encoded surface in much the same manner as a television signal is generated.
However, it is known that the reliability, accuracy, and costs of a sensor are all diminished by moving parts. One way to eliminate a scanning mechanism in an optical sensor is to provide a separate coupler for each code channel and to activate each of the couplers in a sequence corresponding to the significance sequence of the optical code.
Prior art fiber optic technology does provide the means to construct a series of individual couplers, each of which is capable of coupling a respective bit of an optical position code into a single fiber. Such prior art couplers include, for example, bi-conical tapered couplers and evanescent couplers. However, each of these couplers has a significant size which, when replicated by the number of code bits, make miniaturization of an optical sensor impractical.
Other prior art couplers contemplate the severing of an optical fiber, the insertion of a coupling mechanism between the severed ends, and the attachment of the severed ends to the coupling mechanism. These couplers, however, introduce significant excess losses into the optical fiber, and amplify the complexity and cost of sensor manufacture.
Therefore, there is an evident need for an optical coupler design which will operate to efficiently couple an optical signal into an optical fiber, yet which yields a coupler that is small and easy to manufacture.